This invention generally relates to electronic systems and in particular it relates to a class AB input stage.
As the communications technology progresses into higher frequency bands, the requirements for speed in the analog front and back end increase. Operational Amplifier requirements for these applications are not only for a high small signals bandwidth, but also for a high large signals (dynamic) bandwidth. These new Operational Amplifiers need to be even faster than their predecessors in order to ensure low harmonic distortion at high speed.
Large signals bandwidth (BW), or full power bandwidth, is defined as: BW=Slew Rate/2*Vp, where Vp is peak voltage. In other words, the faster the slew rate of the Op Amp, the larger the large signals bandwidth. This also brings better distortion for large signals at high frequencies, mainly due to the fact that the Op Amp will be able to follow faster input signals before it becomes slew rate limited. It is also important to remember that the Op Amp must be power efficient, thus, preferably, only boosting its slewing current during the slewing transitions.
The typical prior art solution to these previously mentioned needs is the Class AB input stage. This topology has the advantage of consuming very low standing current (in absence of input signal or for a small signals input) but capable of boosting its slewing current in the presence of a xe2x80x9clarge signalsxe2x80x9d input stimulus.
The limitation of the conventional Class AB bipolar input stage is a base current limitation that translates into the well known non-linearities that typically are associated with this type of input stage. In other words, there is trade off, when setting this input stage, between available base current to drive the slewing transistor and standing current through those slewing transistors. The boosted slewing current is proportional to the input signal seen by the input stage. Typically the best setting has a low quiescent current and becomes base current limited half way through the slewing transition of the largest possible signal swing. Even with this set up the standing current ends up being substantial which is very inefficient, especially in the case of input signals smaller than the full dynamic input range of the Op Amp. The lack of base current to the slewing transistors translates to distortion, due to the time that the slewing transistor takes to turn xe2x80x9conxe2x80x9d and return back to its quiescent biasing point after suddenly turning xe2x80x9coffxe2x80x9d.
An example of a prior art class AB input stage is shown in FIG. 1. The circuit of FIG. 1 includes transistors 20-31; current sources 34-37; resistors 40-44; capacitor Cc; input nodes IN+ and INxe2x88x92; source voltages VCC and VEE; and output node 46. Resistors 41-44 have the same value. The slew rate (SR) for the prior art circuit of FIG. 1 is given by the following equation:   SR  =            "LeftBracketingBar"                        V                      in            +                          -                  V                      in            -                              "RightBracketingBar"                      R        2            ·              C        c            
Vin+ is the voltage at node IN+. Vinxe2x88x92 is the voltage at node INxe2x88x92. R2 is the resistance of resistor 40. Notice that R2 sets the transconductance (gm) of the input stage, which sets the small signals bandwidth, the open loop gain, noise, and large signals bandwidth. R2 is typically set to be around 500 ohms. For a xc2x110V signal the slewing current is as much as 20V/500 ohm=40 mA. For a one volt step this current would be {fraction (1/500)}=2 mA.
To prevent the non-linearities associated with transistors saturating or turning off during slewing, transistors 20-27 cannot be allowed to ever turn off or even get extremely debiased during slewing. For a 20V step and 40 mA slewing current and beta of 80, the current I must be at least 500 mA, preferably twice that. A 1 mA standing current is wasted if the application never requires the opamp to amplify a signal larger than 1 Vpp, where Vpp is peak-to-peak voltage.
An improved Class AB input stage monitors the needs of base current in the slewing transistors and supplies that base current in an extremely fast and precise feedback loop. This allows the input stage quiescent current to be very small and gets rid of the non-linearities associated with the lack of base current available to drive the slewing transistors in a conventional prior art Class AB input stage. A very efficient, low distortion, high small signals and full power bandwidth Class AB input stage is provided.